The Research Plan identifies the physiologic mechanisms that are involved in the pathogenesis of hydrops fetalis. It is shown that these mechanisms interact in such a way as to make inprobable the current hypotheses of its pathophysiology. A new pathophysiologic mechanism is proposed "autoregulatory edema". The experiments described in the Research Plan deal with the regulation and deregulation of fetal interstitial fluid volumes and pressures. It is proposed to measure these volumes and pressures in guinea pig fetuses with indwelling instrumentation. It is demonstrated that the experimental results can help us distinguish between a variety of new and current hypotheses on the pathophysiology of hydrops fetalis. The experimentally determined parameters will further be of use in a model description of the system. A protocol for human studies is designed to verify the applicability of results obtained in animal work to human pathology.